This insanity bar WA shows the current insanity, the predicted insanity increase. It is part of my main shadow priest WA. (https://wago.io/jfkIfS8yn)
The purple bar is the current insanity, and the brown bar is the predicted insanity.
When the bars are desaturated, it means you don't have enough insanity to use Devouring Plague, or Searing Nightmare (if you have the talent and currently channeling Mind Sear).
The prediction will show you the insanity increase when casting or channeling a spell.
It can dynamically update itself when hitting different number of enemy with Mind Sear.